Isotherms??

[MarxSchmarx]

WTF is an isotherm? I came across this reading something the other day, and googled it. The definition was:

"An isotherm is a line or surface that connects points of equal atmospheric temperature."

So what would be an example? Is it those lines I see on a map during a weather report? Utterly confused, do help!

[genstrike]

Have you ever read a topographic map? It is basically like those, except with temperature instead of altitude.

For example, look at this pic:
http://www.middleschoolscience.com/200306100000_N.gif

The isotherms are the labeled black lines separating the coloured areas. The temperature is constant along those lines (say, 20 degrees on one line, 25 on the next). If you move perpendicular to the isotherms, you are either getting hotter or colder, depending if you are moving towards the next hottest one or the next coldest.

Basically, you can take the temperature at a bunch of points with weather stations, interpolate, and connect the dots to draw the isotherm (or Isobar if you're dealing with pressure), thus giving you a map of temperature over a whole region

hope this helps!

[mikelepore]

I believe the word "isotherm" just means a curve connecting all points of a common temperature, without the word itself telling us what context is intended.

In terms of the atmosphere and weather, when genstrike said -- I wasn't familiar with that use of the word. Thanks - it's interesting to learn about that.

I'm mainly familiar with a different use of the word "isotherm", as a name for a graph which describes a certain kind of thermodynamic process. You have a gas inside of a sealed container. The container is thermostat-controlled so you can keep the temperature inside of it constant, that is, you're going to produce an isothermal process. You have a way to adjust the volume of the container, such as sliding a piston into or out of a cylinder. Now you very slowly reduce the volume of the container, squeezing the gas into a smaller space, which makes the pressure rise, or oppositely, you slowly increase the volume of the container, allowing the gas to expand, which makes the pressure go down. A graph of pressure versus volume is given for that particular kind of gas. The graph shows how the state of the substance goes from an initial point, pressure P1 and volume V1, to a final point, pressure P2 and volume V2. That curve applies only to the pre-set value of temperature, which is kept constant. On the same graph, you have other pressure-versus-volume curves that apply to different temperatures. So you have a family of curves stacked on the same graph. Each one of the curves on the graph is called an isotherm, because it represents a property of the substance at one specific and constant temperature.

[MarxSchmarx]

Huh, both of those explanations make sense.

I've been trying to reconcile the two, let me think about it some more. If in the interim somebody has a clear connection, please clarify as it seems a little convoluted...

[black magick hustla]

you dont have to reconcile both. isotherm means constant temperature, that is why it can be applied to both contexts. For example, a thermodynamic system, like lets say, a piston, expanding with a constant temperature is said to be undergoing isothermal expansion. there is no such thing as a perfect isothermal expansion, but there are things approaching it.